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Water chemistry analysis of an industrial selective flocculation dispersion hematite ore concentrator plantHoward J.Haselhuhn,Joshua J.Carlson,S.Komar Kawatra ⁎Department of Chemical Engineering,Michigan Technological University,1400Townsend Drive,Houghton,MI 49931,USAa b s t r a c ta r t i c l e i n f o Article history:Received 29March 2011Received in revised form 26September 2011Accepted 2October 2011Available online 8October 2011Keywords:HematiteSelective flocculation dispersion Water chemistryHematite ore selective flocculation-dispersion process water is a complex system of ions and reagents work-ing together to produce a concentrated iron oxide product.The purpose of this study was to determine the process water concentrations of the important ionic species in a selective flocculation –dispersion hematite ore concentrating plant while process conditions were stable.This information was used to provide a detailed water chemistry analysis and to further understand how water quality affects the process.An analysis of the water chemistry of an operating selective flocculation-dispersion iron ore concentration plant has never be-fore been published.The pH of the process water was the most important factor in the selective flocculation and dispersion process because,according to prior studies,it has a direct relationship with the surface chem-istry of the particles (Carlson,2010).The pH also had a direct relationship with the solubilities of both calci-um and magnesium in the process.Water hardness was signi ficantly increased with the addition of calcite/dolomite flux which may cause issues in pelletization and in the reuse water.Soluble iron was removed in the flotation circuit suggesting that it may have been oxidized during flotation or it may have been absorbed by the collector.©2011Elsevier B.V.All rights reserved.1.IntroductionSelective flocculation and dispersion is a method of concentrating the valuable constituents of hematite ore bodies.This is accomplished by adding reagents that either flocculate or disperse the particles in a slurry of ground ore by altering the surface chemistry of the ore par-ticles.This methodology has been used sparingly in the mineral pro-cessing industry due to dif ficulties in controlling the surface chemistry characteristics that affect selectivity and high reagent costs (Department of Energy,2001).For these reasons,it is generally not believed that selective flocculation and dispersion is a cost effec-tive method for concentrating iron ore.A Lake Superior district hematite ore concentrating facility has been effectively using selective flocculation and dispersion for con-centrating hematite ore bodies since 1974.Selective flocculation is the process of choice because of the relatively non-magnetic nature and small liberation size (b 25μm)of the iron minerals (Paananen and Turcotte,1980).For selective flocculation and dispersion methods to work,precise control of the surface chemistry of the par-ticles is required (Carlson,2010).Water chemistry is one of the most important aspects of the process because of its direct relation to the surface properties of particles.Very small changes in water chemistry can have a disastrous effect on the grade and recovery of the process.Many years ago there were studies carried out on comminution in this hematite concentrator to determine the effectiveness of grinding aids.During this study,the flotation froth became ineffective for an unknown reason (Klimpel,1990).It was speculated that this plant failure was due to changes in viscosity or water chemistry.There have been many publications on how water chemistry im-pacts the ef ficacy of the hematite ore selective flocculation dispersion process on a laboratory scale.These publications demonstrate the abilities of common dispersants and flocculants in ideally prepared water samples of known chemistry.The detailed water chemistry of an operating selective flocculation-dispersion hematite ore concen-trating facility has never been reported.The purpose of this study was to determine the process water con-centration of a selective flocculation-dispersion hematite ore concen-trating plant while process conditions were stable to provide a detailed water chemistry analysis and further understand the effects of the flocculants and dispersants used throughout the process.2.Background 2.1.Process flow sheetThe selective flocculation dispersion process for hematite ore con-centration begins with crushing and grinding the hematite ore to the liberation size of the hematite particles.The process flow sheet for the hematite concentrator is shown in Fig.1.Twelve autogenous grinding mills with a cone crusher to crush intermediate products feed 24linesInternational Journal of Mineral Processing 102–103(2012)99–106⁎Corresponding author.Tel.:+19064873132;fax:+19064873213.E-mail address:skkawatra@ (S.K.Kawatra).0301-7516/$–see front matter ©2011Elsevier B.V.All rights reserved.doi:10.1016/j.minpro.2011.10.002Contents lists available at SciVerse ScienceDirectInternational Journal of Mineral Processingj o u rn a l ho m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /i j mi n p r oof pebble mills,cyclones,and deslime thickeners.The deslimed slurry enters twelve flotation lines followed by four concentrate thickeners and slurry tanks.This system feeds eight lines of six disc filters per line for concentrate dewatering.All tailings are sent to two tailings thickeners to recover process water (Keranen,1986).2.2.Grinding and deslimingThe most important step in hematite ore concentration is the deslime process in which fine particles are removed from the ground feed.In the best selective flocculation-dispersion conditions,15to 30%of the feed weight can be removed in the deslime thickeners with only a 5%loss in iron concentration (Colombo,1980).These conditions are obtained by adding the correct types and amounts of flocculants and dispersants.Selective desliming for hematite ore utilizes a combination of several dispersants to disperse siliceous gangue followed by a starch to flocculate hydrated hematite particles (Iwasaki,1989).Sodium hydroxide (Caustic soda)is added during grinding to in-crease the alkalinity (pH between 10.5and 11)(Green and Colombo,1984).This increase in alkalinity is necessary to disperse all fine parti-cles in the slurry.At high pH values,the surface charge of the slurry par-ticles become negative,causing dispersion.Minerals that dissolve at higher pH values add metallic cations,namely calcium and magnesium,to the slurry.These positively charged cations adsorb onto the negative-ly charged surfaces as CaOH +and MgOH +and disrupt the sign and magnitude of the surface charge,inhibiting dispersion (Iwasaki,1989).Glass-H®is added to the process after primary comminution to precipitate calcium metaphosphates and magnesium metaphos-phates.The (Ca,Mg)OH +are removed from the particle surfaces allowing retention of the dispersive qualities of the negatively charged surfaces (Iwasaki,1989).Glass-H®is a trade name for sodi-um hexametaphosphate (SHMP).Starch is added to the deslime thickener feed as a selective floccu-lant for hematite.The starch forms hydrogen bonds with the oxide surfaces of hematite particles and selectively flocculates the hematite particles (Balajee and Iwasaki,1969).The effectiveness of starchadsorption is relative to the mineralogy of the ore and the water qual-ity.An over-abundance of calcium or magnesium ions can cause the particle surfaces to become completely coated in metaphosphate pre-cipitates and hinder starch adsorption and therefore hematite floccu-lation (Iwasaki,1989).2.3.Flotation and dewateringThe deslime thickener over flow is sent for treatment in one of the tailings thickeners.The under flow,which has been increased to 35to 40%iron in the desliming process,is sent to a conditioner where it is once again mixed with starch that coats the hematite particles (Benner and Goetzman,1994).This creates a barrier to both surface charge and Van der Waals forces to eliminate any attraction of the silica to the he-matite.The conditioned slurry feed is fed into the reverse cationic flota-tion circuit consisting of a rougher followed by four stages of scavenger flotation.It is mixed with an 8–10chain primary ether amine that acts as both a cationic collector and frothing agent in the rougher feed box (Keranen,1986).Ether amine and starch are also added to the scaven-ger flotation cells to increase the recovery of the hematite.The concen-trate of each scavenger is fed back into the rougher.The flotation concentrate is pumped to the concentrate thickener where it is thickened and fed into a slurry tank.Pressurized carbon di-oxide is mixed with the slurry at one of 6injection points to lower the pH of the slurry and prepare the slurry for filtration (Keranen,1986).A limestone (CaCO 3)/dolomite (CaMg(CO 3)2)flux is added to the slurry tank to improve reduction properties of the iron in the blast furnace (Benner and Goetzman,1994).The fluxed slurry is filtered using vacuum disc filters.The fluxed hematite filter cake is then sent for pelletization.3.Materials and methodsSamples were taken at 11sampling locations throughout the he-matite concentrator.Fig.2shows the general layout of the hematite concentrating plant and the sampling locations.Each locationhasFig.1.Hematite ore concentrator circuit as described by Keranen (1986).100H.J.Haselhuhn et al./International Journal of Mineral Processing 102–103(2012)99–106been assigned a number for ease of data presentation.Table 1shows descriptions for each sampling location along with their assigned numbers for future reference.All water chemistry data presented will refer to these location numbers.The samples were taken in 1gal plastic buckets and brought back to Michigan Technological University immediately after being taken.The supernatant fluid from each sample was decanted into 1L glass sample bottles and tested for conductivity and pH.The 1L samples were kept in a refrigerator and sent in a cooler packed with ice packs to a certi fied laboratory for further testing.Tests were conducted on each sample to determine the concentra-tions of cations (calcium,magnesium,sodium,potassium,and iron),anions (chloride,sulfate,fluoride,nitrate,nitrite,carbonate and bicar-bonate),hardness,total organic carbon (TOC),Kjeldahl nitrogen,and grease/oil.pH was also re-tested at the certi fied laboratory to con firm the results obtained at Michigan Technological University and to de-termine if there is potential for pH drift over time.4.Results and discussionTest results were received from the laboratory detailing the water chemistry of the hematite ore concentrating facility.4.1.pHFig.3shows the results of the pH tests conducted at Michigan Technological University's Advanced Sustainable Iron and Steel Mak-ing Center (ASISC)and at the certi fied laboratory.pH played a signif-icant role in the removal of silica in the deslime thickener and in flotation.Fine silica and other gangue minerals were removed in the deslime thickener at a pH of around 11.This pH was necessary be-cause hematite particles exhibit a net negative surface charge allow-ing for a near complete dispersion of the fine silica particles from the hematite surfaces.Most of the remaining silica particles were removed in the flota-tion process.This was kept at a pH of 11to maintain the negative sur-face charge of silica and maximize the effectiveness of the cationic amine collector.After flotation,the concentrate entered the concen-trate thickener,where it was thickened prior to filtration.The concen-trate thickener under flow was sparged with CO 2.The addition of CO 2caused a drop in pH,yielding a decrease in surface charge of the he-matite particles.This caused silica fines to flocculate with hematiteparticles increasing the effective particle size and therefore filtration rate (Esumi et al.,1988).A pH drift was noticed for these samples over the course of several days.The pH of each sample tended to decrease slowly with time as shown by the higher pH results obtained while the samples were at Michigan Technological University prior to shipment to the certi fied laboratory.4.2.ConductivityFig.4shows the conductivity of the process water by location throughout the hematite ore concentrating process.The conductivity remained consistent between 1907and 2100μS/cm (μΩ−1cm −1)until the addition of flux where the conductivity increased to 2389μS/cm.This may have been due to the dissolution of the lime-stone and dolomite in the flux and the dissolved carbonate/bicarbon-ate ions present from the addition of carbon dioxide.4.3.CalciumCalcium concentration after primary comminution was found to be 4.47mg/L at location one prior to Glass-H®addition,see Fig.5.Glass-H®is typically added to a slurry as a dispersant,in this case,to form calcium and magnesium precipitates and remove multivalent cations from the surface of the slurry particles (Iwasaki,1989).This allows for greater dispersion of the negatively charged particles.Calcium concentrations above 25mg/L in the flotation circuit are known to be detrimental to iron recovery (Benner and Goetzman,Fig.2.Hematite concentrator process flow sheet with water analysis sample locations.Each numbered circle represents a sample location and data is characterized based on these sample location numbers.Table 1Sampling location descriptions.Location Description1Screen under flow before glass –h addition 2Screen under flow after glass –h addition 3Cyclone over flow4Deslime thickener over flow 5Deslime thickener under flow 6Flotation feed 7Scavenger returns 8Scavenger tails9Rougher concentrate10Concentrate thickener under flow11Filter feed after CO 2and flux addition101H.J.Haselhuhn et al./International Journal of Mineral Processing 102–103(2012)99–1061994).This is caused by ionic competition of the cationic collector (Iwasaki,1989).These concentrations were averted by using Glass-H®in the desliming process.The calcium concentration throughout the flotation circuit was found to be between 4and 10mg/L.The calcium concentration remained low until the addition of the limestone/dolomite flux.The flux addition increased the calcium con-centration to 71mg/L showing that some of the limestone and dolomite dissolved into the solution.This could be detrimental to downstream processes in which bentonites are used to bind the concentrated hema-tite ore into pellets.Calcium displaces the sodium in bentonite as an ex-changeable cation which converts the bentonite from a highly active sodium bentonite into a relatively inactive calcium bentonite.It has been shown that the calcium concentration of water extracted from a filtered hematite ore concentrate can be as high as 550times that of the calcium concentration found in the filtrate (Ripke and Kawatra,2003).This increase in concentration just prior to filtration could be ex-tremely detrimental to the pelletization process.4.4.MagnesiumMagnesium,like calcium,is a multivalent cation that plays a sig-ni ficant role in the dispersion of hematite from silica.The entering concentration of magnesium into the concentrator was 2.67mg/L,see Fig.6.The deslime thickener was operated at a magnesium con-centration of about 5mg/L.The magnesium concentration dropped to nearly zero throughout the flotation process suggesting that the ether amine absorbs the magnesium.Iwasaki (1989)showed that flo-tation recovery dropped abruptly to nearly zero with magnesium concentrations above 10−3M (~24mg/L)which supports this hy-pothesis.If the amount of collector added is not enough to absorball of the magnesium,magnesium hydroxide precipitates on the quartz surfaces,hindering flotation (Iwasaki,1989).The magnesium concentration increased to 58mg/L after the addition of flux.This was most likely due to the dissociation of magnesium from dolomite in the flux.4.5.SodiumTable 2shows the sodium concentration by location throughout the hematite ore concentrating process.The entering sodium concen-tration was 455mg/L and increased with the addition of Glass-H®.This was to be expected because the sodium in Glass-H®(sodium hexametaphosphate)was exchanged with calcium and magnesium ions in solution.The deslime thickener seemed to remove some sodi-um along with silica at location 4;there was a 32mg/L concentration difference between the sodium concentration of the over flows and under flows.This suggests that sodium left the process adsorbed to the negatively charged silica surfaces.The concentrate thickener seemed to concentrate the sodium as well by increasing the concentration by 27mg/L.This also suggests that the sodium can be found near the negatively charged particle surfaces.The addition of CO 2and flux also caused an increase in sodi-um concentration by 35mg/L most likely due to impurities in the limestone/dolomite flux.4.6.PotassiumMonovalent ionic species such as potassium are less likely to cause harmful effects on flocculation-dispersion processes;howeverFig.3.pH of process water by location.pH remains stable throughout process until in-troduction of CO 2and flux at filter feed (Location 11in Fig.2).Fig.4.Conductivity of process water by location.Conductivity increases signi ficantly with the addition of flux at filter feed (Location 11in Fig.2).Fig.5.Calcium concentration of process water by location.Calcium concentration in-creases signi ficantly at filter feed suggesting that flux is dissolving (Location 11in Fig.2).Fig.6.Magnesium concentration of process water by location.Magnesium concentra-tion increases signi ficantly at filter feed suggesting that dolomite is dissolving (Loca-tion 11in Fig.2).102H.J.Haselhuhn et al./International Journal of Mineral Processing 102–103(2012)99–106potassium fluoride has been shown to inhibit the activation of quartz particles in selective flocculation –dispersion processes of hematite (Attia,1982).Fig.7shows the potassium concentration by location throughout the hematite ore concentrating process.The entering po-tassium concentration was 11.4mg/L after autogenous grinding.The addition CO 2and flux nearly doubled the concentration of potassium,likely due to impurities in the flux.4.7.Soluble ironFig.8shows the soluble iron concentration by location throughout the hematite ore concentrating process.The entering concentration of soluble iron was 16.4mg/L.The soluble iron concentrations slowly in-creased throughout the grinding and deslime process.It immediately dropped to near zero in the flotation circuit.This suggests that the ether amine cationic collector may have absorbed the soluble iron species in solution or the soluble iron was oxidizing in the flotation circuit.The soluble iron concentration remains less than 1mg/L at every location downstream of the flotation circuit.4.8.ChlorideTable 2shows the chloride concentration by location throughout the hematite ore concentrating process.As shown,the chloride con-centration did not change by more than 10mg/L throughout the en-tire process.4.9.FluorideFluorides present in the water can be attributed to the grinding of a fluorapatite.These fluorides are readily precipitated as calcium fluo-ride and were not prevalent in the concentrator water chemistry analysis.Table 2shows the fluoride concentration by location throughout the hematite ore concentrating process.It has been shown that fluoride concentration is limited by the presence of calci-um and is promoted by the presence of sodium (Engesser,2003).All three of these ions were present throughout the concentrator at con-sistent concentrations and no relationship could be adequately deter-mined based on this data.4.10.SulfateTable 2shows the sulfate concentration by location throughout the hematite ore concentrating process.The entering sulfate concentration was 90mg/L after autogenous grinding and progressively lowered throughout the plant to a minimum of 83mg/L at the concentrate thick-ener feed.The addition of a sulfosuccinate tailings treatment at the con-centrate thickener feed may have increased the concentration of sulfates in solution to 110mg/L (Keranen,1986).The concentration was then lowered to 100mg/L following the addition of CO 2and flux.4.11.NitrateFig.9shows the nitrate concentration by location throughout the hematite ore concentrating process.The entering nitrate concentrationTable 2Sodium,chloride,fluoride,and sulfate concentration of process water by location.Con-centrations remain relatively stable throughout process.Sulfate concentration has a slight increase upon flux addition.LocationSodium (mg/L)Chloride (mg/L)Sulfate (mg/L)Fluoride (mg/L)145511090 4.12465110904345411088 3.9447811088 3.75446110894644811088 3.8743710085 3.8845910085 3.8943310083 3.810460110110 5.511495100100 4.2Average457106.91 4.1Standard deviation17.75.07.80.5Fig.7.Potassium concentration of process water by location.Potassium concentration remains stable with a spike at filter feed (Location 11in Fig.2)possibly due to impuri-ties in flux.Fig.8.Soluble iron concentration of process water by location.Soluble iron steadily in-creases until flotation (Location 7in Fig.2),where it either binds with the collector or isoxidized.Fig.9.Nitrate concentration of process water by location.Nitrate concentration re-mains stable until filter feed (Location 11in Fig.2)where the nitrates may be affected by the change in pH.103H.J.Haselhuhn et al./International Journal of Mineral Processing 102–103(2012)99–106was 16mg/L and remained fairly constant throughout the process until CO 2and flux were added between locations 10and 11.4.12.NitriteFig.10shows the nitrite concentration by location throughout the hematite ore concentrating process.The nitrite concentration remained almost undetectably low (bb 1mg/L)throughout the process.4.13.Carbonate and bicarbonateFigs.11and 12show the carbonate and bicarbonate concentra-tion by location throughout the hematite ore concentrating process.The carbonate and bicarbonate concentration deviated opposite to each-other throughout the process suggesting that they are in equilibrium with each other as shown in Eq.(1).The equilibrium concentrations of these carbonate species are dependent on the pH of the solution.As Fig.13shows,lower pH values (~7–10)cor-respond to higher concentrations of the bicarbonate ion,whereas higher pH values (~10+)correspond to higher concentrations of the carbonate ion.CO 2−3þH 2O ↔HCO −3þOH−HCO −3þH 2O ↔H 2CO 3þOH−(1)Carbonate –Bicarbonate –Carbonic Acid EquilibriumAs seen in Fig.3,after the addition of CO 2the pH dropped consid-erably down to 7.67,explaining the loss of all carbonate ions and theappearance of 1295mg/L of bicarbonate ions in solution.This showed that the process water takes on approximately 550mg/L of carbonate ions as a result of the forced injection of CO 2and the addition of the limestone/dolomite flux.Literature states that the solubilities of both limestone and dolo-mite increase with a decrease in pH (Cowan and Weintritt,1976).The presence of carbonate species in solution slightly lowers the sol-ubility of calcium carbonate.The relationship between calcium car-bonate solubility,pH and carbonate concentration is shown on the surface plot in Fig.14.The decreased flux solubility due to the pres-ence of more carbonate ions was not enough to counteract the in-crease in solubility due to the drop in pH.This explains the increase in calcium and magnesium concentration seen upon flux addition.As stated in Sections 4.3and 4.4,an increase in the calcium or magne-sium concentration can be detrimental to downstream processes.4.14.HardnessFig.15shows the hardness of the process water by location throughout the hematite ore concentrating process.The entering hardness after comminution was 22mg/L and generally followed the trend of both calcium and magnesium.The water remained rela-tively soft throughout the process until the addition of CO 2and flux.A sample of the process water at the concentrate thickener under-flow was sparged with CO 2until the solution was saturated.The water analysis of this sample was compared to the samples at locations 9and 11to show how CO 2affects the water chemistry with regardstoFig.10.Nitrite concentration of process water by location.Nitrite concentration re-mains extremely low and possibly at the threshold ofdetectability.Fig.11.Carbonate concentration of process water by location.Carbonate concentration remains stable until filter feed (Location 11in Fig.2)when the change in pH causes all carbonates to hydrolyze tobicarbonates.Fig.12.Bicarbonate concentration of process water by location.Bicarbonates remain at near zero until the pH is adjusted with an addition of CO 2(Location 11in Fig.2).The bicarbonate concentration at location 11includes all hydrolyzed carbonates,dissolved CO 2,and dissolved flux.Fig.13.Concentration of H x CO 3ions with respect to the total CO 3concentration (C T )as a function of pH (Zasoski,2001).104H.J.Haselhuhn et al./International Journal of Mineral Processing 102–103(2012)99–106carbonate,bicarbonate and hardness concentrations.The results of this test are shown in Table 3.With no CO 2or flux added,the pH was highenough to retain all the carbonate concentration as CO 32-.With the addi-tion of only CO 2,the pH dropped and carbonate equilibrium shifted tothe bicarbonate (HCO 3-)species.Some change was noticed in the hard-ness suggesting that some calcium may be dissolved into the solution from particles surfaces at the lower pH.Addition of the limestone/dolomite flux increased both the bicarbonate concentration and the hardness of the process water.This is another positive indication that some of the flux was dissolving in the slurry.4.15.Total organic carbonFig.16shows the total organic carbon (TOC)concentration by lo-cation throughout the hematite ore concentrating process.The TOCremained fairly low throughout the process until the flux and CO 2are added at location 11.The hematite particle charge reversal seen at the lowered pH at location 11may have released adsorbed posi-tively charged organic carbons from hematite particle surfaces caus-ing the spike to 113mg/L of total organic carbon.4.16.Kjeldahl nitrogenKjeldahl nitrogen refers to a testing method used for determining the organic nitrogen,amine,and ammonium concentration of a solu-tion.Fig.17shows that there was no increase in nitrogen concentra-tion until the addition of the ether amine cationic collector in the flotation circuit.The rougher concentrate contained 5.12mg/L of ni-trogen possibly from a cationic exchange of soluble iron with the amine collector.The excess nitrogen was removed in the concentrate thickener over flow.The nitrogen concentration increased again with the addition of CO 2and flux indicating that the change in pH may have released cationic nitrogen based species from particle surfaces;similar to the effect seen with organic carbons.4.17.Oil/greaseFig.18shows the oil and grease concentration by location throughout the hematite ore concentrating process.This may give clues to areas where oil and grease may be being added unintention-ally from bearings or other mechanical devices.It appeared that the presence of oil and grease in the process water was sporadic and does not consistently correlate to reagentadditions.Fig.14.Calcium carbonate solubility as a function of both pH and dissolvedcarbonates.Fig.15.Hardness of process water by location.Hardness peaks at filter feed (Location 11in Fig.2)suggesting the dissolution of flux.Table 3Water analysis comparison of concentrate thickener under flow (Location 10in Fig.2)reagent additions.CO 2injection alone cannot create the difference in bicarbonate con-centration and water hardness;flux must be dissolving to some extent.Concentrate thickener under flow addition Carbonateconcentration (mg/L)Bicarbonateconcentration (mg/L)Hardness (mg/L)None 752021CO 2082652CO 2and flux1295416Fig.16.Total organic carbon (TOC)concentration of process water by location.Change in surface charge of hematite particles may be releasing organic carbons from surface at location 11in Fig.2.Fig.17.Kjeldahl nitrogen testing results for process water by location.Ether amine cat-ionic collector may be leaving in rougher concentrate (Location 9in Fig.2).Ether amine may also be desorbed from hematite surfaces at lower pH at filter feed (Location 11in Fig.2).105H.J.Haselhuhn et al./International Journal of Mineral Processing 102–103(2012)99–106。